The present invention relates to a fluid-bed drying unit preferably but not exclusively used in the field of tobacco drying.
It is known that drying requires the control of all the various factors involved in this kind of process. In particular, the rate of transit or flow rate of the products through the drying chamber must be adjustable so as to keep the product therein for the time required for complete drying.
Furthermore it is necessary to remove the moisture produced by the process and to control the temperature and relative humidity of the drying fluid used.
Several kinds of drying units, which use different techniques, are known for this purpose in the field. For example, convection drying units are commercially available in which drying occurs by means of the heat exchange of a fluid which passes through the product to be dried, while conveyance occurs by way of a mechanical means (for example moving conveyors of the belt type or perforated-strip type, or vibrating conveyors).
Other conventional types of drying units are those which use combined conduction/convection, for example inside rotating cylinders or screw-feeder conveyors, in which drying occurs through the contact of the product against a warm surface of the conveyance means (for example the warm walls of the conveyor means) and by convection by way of a fluid which flows by the product and removes the moisture produced during the process.
Finally, so-called fluid-bed drying units are known whose characteristic is that they use only the drying fluid both for drying and for conveyance. In this case, the drying fluid flows over and through the product, warming it, and at the same time conveying it toward the outlet of the drying chamber.
A distinction is made between fluid-bed drying units with a nil fluid/product relative velocity (i.e., the fluid conveys the product at the speed at which the fluid itself travels) and fluid-bed drying units with a nonzero fluid/product velocity (i.e., the product is caused to float by the fluid during drying, and in the subsequent step the fluid conveys the product toward the outlet with nil fluid/product velocity).
These commercially known devices suffer many drawbacks. In particular, in the case of zero-velocity drying units, the length of the drying chamber must be great enough to allow the retention time of the product to be sufficient for complete drying. This implies considerable dimensions, which as such entail very high costs, and reduces the possibility to control the different steps of the process. These problems are even more strongly felt in the case of a soft drying process, in which the necessary length becomes almost impracticable.
Another drawback of this conventional drying unit relates to the distribution of the product inside the drying chamber. If the travel velocity of the product is constantly determined by the velocity of the fluid, a poor initial distribution of the product cannot be corrected during the process, consequently limiting the uniformity and quality of the drying action.
Yet another drawback of the existing drying units is that their drying chambers must also have rather significant height extensions, such as to provide a homogeneous dispersion of the product within the fluid. This also contributes to increase the bulk of the now available units.
Moreover, in the case of drying with a nonzero fluid/product velocity, there can be parts having a different mass or surface floating differently during drying. This entails a dependency between the mass/surface of the product and the exposure time.
The result is a difficult adjustment of the drying times and of the temperature of the fluid, with corresponding technical problems in the distribution of the temperature inside the drying chamber.
It is well-known that this type of drying is not suitable for flash drying, for which adjustment of the flotation and of the temperature is even more difficult.